If the default encoding in your source code repository is not UTF-8, you can specify it in the trac.ini, for example:

default_charset = gbk

You also must make sure that your ​database backend stores its data in UTF-8; otherwise strange things will happen.

To convert your database to UTF-8, the easiest way is to dump the database, convert the dump into UTF-8 and then import the converted dump back into the database.
You can use ​iconv to convert the dump.